Asynchronous transfer mode (ATM) is rapidly becoming the technology of choice for broadband information transport. The ability of an ATM network to effectively transmit voice, video and data has played a major role in the wide acceptance of the technology. Of particular importance is the ability of ATM to carry bandwidth on demand (BOD) traffic which is typified by today's local area network (LAN) traffic. In light of applications such as these the concept of rate-based flow control is introduced to ensure the BOD traffic can retain its inherent bursty nature and yet allow for an efficient usage of network resources. One such rate-based flow control scheme as adopted by the ATM Forum is the available bit rate (ABR) service category. It is therefore important for ATM switches to support such ABR traffic to ensure a high performance in switch capacity utilization.
The ABR rate-based flow control scheme is based on a feedback control loop which utilizes bi-directional cell flows for traffic management. Since ATM cell traffic is comprised of various service categories with different quality of service guarantees, network elements are required to switch traffic streams through nodes in the switch fabric. This traffic control results in the creation of queuing points within the network elements in order to accommodate the bursty nature of traffic streams. To accommodate ABR rate-based traffic while optimizing utilization of available bandwidth, mechanisms are required to detect potential congestion at these queuing points within the network elements. ABR traffic streams include periodically transmitted resource management cells which are marked with congestion information. The resource management (RM) cells travel through network elements between the network source to the network destination, the resource management cells being marked with congestion information en route. At the destination the ABR traffic including resource management cells are returned to the source where the congestion information is used to control the flow rate of subsequent cell traffic. The feedback information can be in the form of an explicit rate (ER) marking or a congestion indicating bit (relative rate marking) as specified by the ATM Forum. The congestion information, as indicated previously, is carried in both a forward and backward direction. Being a feedback control system in nature, large network delays would tend to degrade the performance of ABR traffic in terms of steady-state utilization and convergence time. In order to reduce network delay the concept of virtual source, virtual destination (VSVD) has been introduced. This concept segregates an otherwise long feedback control loop into multiple shorter loops thus allowing a better overall utilization of network bandwidth.
Current VSVD architectures call for brute force implementation using high computational power RISC-based processors and/or specially designed digital hardware (VLSI). Such centralized solutions are of high complexity, power, cost and usually do not scale well.